Anyone had this? Popped up last night. Not long after setting off, light throttle, it was fairly cold. Car appeared to drive normally, but I didn’t check for reduced power. Didn’t return after restarting the car.
Did it happen when the heater came on - either by pressing the switch or as a result of the climate control turning it on? I had this behaviour (see Odd Heater fault) and it turned out to be due to a faulty three way valve in the cooling system. If it is the same fault, it doesn't then happen after turning off and then on because the heater is already activated at switch on (from when you turned the car off previously) so presumably the valve doesn't have to open. The way you can test is manually turn off the heater and then turn it on again a few seconds later, in my case the fault only happened when I was driving and only when the heater actually switches on.
Given that this is a fairly general warning light, it may be of course that in your case it's due to something completely different, but it's worth checking if it is heater switch-on related. I too encountered no effect on the drive systems and have been on two 250 mile plus trips since it was fixed without further incident.